This invention relates to video processing.
Video is principally a series of still pictures, one shown after another in rapid succession, to give a viewer the illusion of motion. In many computer-based and network-based applications, video plays important roles. Before it can be transmitted over a communication channel, video may need to be converted, or “encoded,” into a digital form. In digital form, the video data is made up of a series of bits called a “bitstream.” Once encoded as a bitstream, video data may be transmitted along a digital communication channel. When the bitstream arrives at the receiving location, the video data are “decoded,” that is, converted back to a form in which the video may be viewed. Due to bandwidth constraints of communication channels, video data are often “compressed” prior to transmission on a communication channel. Compression may result in a loss of picture quality at the receiving end.
A compression technique that partially compensates for loss of quality involves separating the video data into two bodies of data prior to transmission: a “base layer” and one or more “enhancement layers.” The base layer includes a rough version of the video sequence and may be transmitted using comparatively little bandwidth. Each enhancement layer also requires little bandwidth, and one or more enhancement layers may be transmitted at the same time as the base layer. At the receiving end, the base layer may be recombined with the enhancement layers during the decoding process. The enhancement layers provide correction to the base layer, consequently improving the quality of the output video. Transmitting more enhancement layers produces better output video, but requires more bandwidth. A bandwidth is often measured in units of bits per second (bps) or bytes per second (Bps). A communication channel provided by an ordinary telephone line, for example, may have a bandwidth of 28.8 kbps, or 28,800 bits per second. In some circumstances, bandwidth restrictions may prevent transmitting the full number of enhancement layers.
Enhancement layers may contain information to enhance the color of a region of a picture and to enhance the detail of the region of a picture.